A HISTORY OF SUSSEX 



links are discovered species pass by imperceptible gradations into each 

 other, so that the ordinary rules of zoological nomenclature break down, 

 and it is exceedingly difficult to know what to call any but the extreme 

 forms. Space will not permit any discussion of these variable fossils ; 

 we can only observe that they are particularly abundant in the lower 

 part of the Upper Chalk, Micraster cor-bovis being confined to the 

 Holaster planus zone and the Chalk below ; while Micraster cor-testu- 

 dinarium and M. cor-anguinum give their names to the next two zones, 

 respectively no and 240 feet in thickness. Though still-varying 

 micrasters extend throughout the Upper Chalk, other more abundant 

 fossils have been selected to give their names to the higher zones. Next 

 comes softer chalk with fewer flints, at any rate inland, belonging to the 

 zone of Marsupites, curious purse-like echinoderms, two species of which 

 are not uncommon, associated with JJintacrinus and the small sea urchin 

 Cardtaster pilula. On the coast the thickness of this zone is nearly 

 80 feet. 



The next zone, that of Actinocamax quadratus, is more flinty, and 

 this superposition of flinty chalk on soft, easily eroded chalk with few 

 flints has led to the formation of a series of isolated, flat-topped hills, with 

 steep scarps towards the north and gentler slopes to the south, which 

 form a chain extending east and west through the middle of the Downs. 

 Each of these hills was fortified in prehistoric times, the best known of 

 the fortresses being the conspicuous camp of Cissbury, where also the 

 flints of this zone were mined for the manufacture of implements. 

 According to Dr. Rowe, a thickness of 170 feet of Chalk belonging to 

 this zone can be measured in the cliffs. It is doubtful whether there is 

 more than this anywhere in the county, for the higher part of the zone 

 and the whole of the zones above appear to be missing. 



The best places to study the Chalk zones in Sussex are, for the 

 Middle and Upper Chalk, the cliffs between Eastbourne and Brighton ; 

 but for the Lower Chalk the large pits near Lewes are more satisfactory, 

 for at Eastbourne the Lower Chalk is exceptionally hardened and dis- 

 turbed. With regard to the zones into which the Chalk is divided, it 

 may be remarked that these are belts corresponding roughly with the 

 occurrence of certain faunas, and that the name of a particular fossil is 

 only given to a zone as a matter of convenience. The name-fossil may 

 be entirely absent from a particular district, or it may there range above 

 or below the zone to which it gives its name ; and usually, as might be 

 expected, most of the other species overlap towards the border and are 

 found also in the zone above or below. Hard and fast boundaries are no 

 more to be found between life-zones, except in rare cases, than they are 

 to be found between species ; it is only our imperfect knowledge that in 

 each case has allowed us to draw sharp lines. 



Between the Chalk of Sussex and the oldest of the overlying 

 Tertiary deposits there exists an enormous gap. Not only is great part 

 of the Chalk missing, but several early Eocene deposits elsewhere well 

 developed and containing peculiar fiunas are absent ; even the Thanet 



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